The human knee is a complex joint containing spatially interrelated bones, ligaments, and cartilaginous structures which interact to create a variety of motions. Specifically, the femoral condyles articulate with the surface plateaus of the tibia, through the cartilaginous medial and lateral menisci, and all of these structures are held in place by various ligaments. By virtue of their cartilaginous nature, the medial and lateral menisci are structures comprised of cells called fibrochondrocytes, an interstitial matrix of fibers of the protein collagen, and within a ground substance formed from proteoglycans. Undamaged menisci provide shock absorption for the knee by ensuring proper force distribution, stabilization, and lubrication for the interacting bone surfaces within the knee joint, which are routinely exposed to repeated compression loading during normal activity. Much of the shock absorbing function of the medial and lateral menisci is derived from the elastic properties inherent to cartilage. When menisci are damaged through injury, disease, or inflammation, arthritic changes occur in the knee joint, with consequent loss of function.
Since joint cartilage in adults does not naturally regenerate to a significant degree once it is destroyed, damaged adult menisci have historically been treated by a variety of surgical interventions including removal and replacement with prosthetic devices. An artificial knee joint having a rigid plastic femoral member and a metal tibial member is disclosed in U.S. Pat. No. 4,034,418. A number of meniscus prostheses have been devised which employ resilient materials such as silicone rubber or natural rubber, as in U.S. Pat. No. 4,344,193 and U.S. Pat. No. 4,502,161. Additional deformable, flexible resilient materials for a meniscus prosthesis such as collagen, tendon, or fibrocartilage are disclosed in U.S. Pat. No. 5,092,894 and U.S. Pat. No. 5,171,322. A cartilage replacement apparatus constructed of polyethylene plastic filled with small ball bearings or gelatinous fluid is described in U.S. Pat. No. 5,358,525. However, the known artificial prostheses have been unsatisfactory for treatment of damaged menisci, since they are deficient in the elastic, and therefore in the shock-absorbing, properties characteristic of natural menisci. Moreover, the known artificial devices have not proven able to withstand the forces inherent to routine knee joint function.
The present inventor provided improved prosthetic menisci in several of his earlier patents (U.S. Pat. No. 4,880,429; U.S. Pat. No. 5,007,934; U.S. Pat. No. 5,116,374; and U.S. Pat. No. 5,158,574). These patents generally disclose prosthetic menisci formulated from dry, porous matrices of processed natural fibers such as reconstituted cross-linked collagen, which optionally include glycosaminoglycan molecules. Generally, the source of collagen for these prosthetic menisci has been animal Achilles tendons or skin. The reconstitution process removes non-collagenous materials such as glycoproteins, proteoglycans, lipids, native glycosaminoglycans, and the like, which may confer additional elastic properties to the original tissue.
Much of the structure and many of the properties of original tissues may be retained in transplants through use of xenogeneic or heterograft materials, that is, tissue from a different species than the graft recipient. For example, tendons or ligaments from cows or other animals are covered with a synthetic mesh and transplanted into a heterologous host in U.S. Pat. No. 4,400,833. Flat tissues such as pig pericardia are also disclosed as being suitable for heterologous transplantation in U.S. Pat. No. 4,400,833. Bovine peritoneum fabricated into a biomaterial suitable for prosthetic heart valves, vascular grafts, burn and other wound dressings is disclosed in U.S. Pat. No. 4,755,593. Bovine, ovine, or porcine blood vessel heterografts are disclosed in WO 84/03036. However, none of these disclosures describe the use of a xenograft for meniscus replacement.
Xenograft materials must be chemically treated to reduce immunogenicity prior to implantation into a recipient. For example, glutaraldehyde is used to cross-link or "tan" xenograft tissue in order to reduce its antigenicity, as described in detail in U.S. Pat. No. 4,755,593. Other agents such as aliphatic and aromatic diamine compounds may provide additional crosslinking through the sidechain carboxyl groups of aspartic and glutamic acid residues of the collagen polypeptide. Glutaraldehyde and diamine tanning also increases the stability of the xenograft tissue.
Xenograft tissues may also be subjected to various physical treatments in preparation for implantation. For example, U.S. Pat. No. 4,755,593 discloses subjecting xenograft tissue to mechanical strain by stretching to produce a thinner and stiffer biomaterial for grafting. Tissue for allograft transplantation is commonly cryopreserved to optimize cell viability during storage, as disclosed, for example, in U.S. Pat. No. 5,071,741; U.S. Pat. No. 5,131,850; U.S. Pat. No. 5,160,313; and U.S. Pat. No. 5,171,660. U.S. Pat. No. 5,071,741 discloses that freezing tissues causes mechanical injuries to cells therein because of extracellular or intracellular ice crystal formation and osmotic dehydration.